Added visitor pattern test case

docs/test_cases/t00018.md

@@ -17,6 +17,36 @@ diagrams:
 
 ```
 ## Source code
+File t00018_impl.cc
+```cpp
+#include "t00018_impl.h"
+#include "t00018.h"
+
+namespace clanguml {
+namespace t00018 {
+namespace impl {
+
+widget::widget(int n)
+    : n(n)
+{
+}
+
+void widget::draw(const clanguml::t00018::widget &w) const
+{
+    if (w.shown())
+        std::cout << "drawing a const widget " << n << '\n';
+}
+
+void widget::draw(const clanguml::t00018::widget &w)
+{
+    if (w.shown())
+        std::cout << "drawing a non-const widget " << n << '\n';
+}
+}
+}
+}
+
+```
 File t00018_impl.h
 ```cpp
 #pragma once
@@ -101,36 +131,6 @@ widget &widget::operator=(widget &&) = default;
 }
 }
 
-```
-File t00018_impl.cc
-```cpp
-#include "t00018_impl.h"
-#include "t00018.h"
-
-namespace clanguml {
-namespace t00018 {
-namespace impl {
-
-widget::widget(int n)
-    : n(n)
-{
-}
-
-void widget::draw(const clanguml::t00018::widget &w) const
-{
-    if (w.shown())
-        std::cout << "drawing a const widget " << n << '\n';
-}
-
-void widget::draw(const clanguml::t00018::widget &w)
-{
-    if (w.shown())
-        std::cout << "drawing a non-const widget " << n << '\n';
-}
-}
-}
-}
-
 ```
 ## Generated UML diagrams
 ![t00018_class](./t00018_class.png "Pimpl pattern")